The 3GPP Rel-13 feature “Licensed-Assisted Access” (LAA) allows LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices (i.e., LTE user equipment (UEs)) connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard, also known under its marketing brand as “Wi-Fi.”
In Europe, the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum the LAA LBT procedure shall conform to requirements and minimum behaviors set forth in EN 301.893. However, additional system designs and steps are needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
As an example, U.S. Pat. No. 8,774,209 B2, “Apparatus and method for spectrum sharing using listen-before-talk with quiet periods,” discusses where LBT is adopted by frame-based OFDM systems to determine whether the channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and is followed by a quiet period. In contrast, this invention focuses only on the LBT phase of a load-based OFDM system, and is designed to ensure fairer coexistence with other radio access technologies such as Wi-Fi while also satisfying EN 301.893 regulations.
Long Term Evolution (LTE)
FIG. 1 illustrates the basic LTE downlink physical resource. LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier FDMA (SC-FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
FIG. 2 illustrates the LTE time-domain structure. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame 210 consisting of ten equally-sized subframes of length Tsubframe, 1 ms, in the illustrated example embodiment. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a RB corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent RBs in time direction (1.0 ms) is known as a resource block pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled. For example, in each subframe, the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g., the control information. FIG. 3 illustrates an example downlink subframe with CFI=3 OFDM symbols as control. The reference symbols shown there are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
From LTE Rel-11 onwards, DL or UL resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). By contrast, according to Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available.
Physical Downlink Control Channel and Enhanced Physical Downlink Control Channel
The Physical Downlink Control Channel (PDCCH) and the Enhanced Physical Downlink Control Channel (EPDCCH) may be used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. For example, DCI may include downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-Automatic Repeat Request (HARQ) information, and/or control information related to spatial multiplexing where applicable. A downlink scheduling assignment may also include a command for power control of the PUCCH used for transmission of HARQ acknowledgements in response to downlink scheduling assignments. Additionally or alternatively, DCI may include uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and HARQ-related information. An uplink scheduling grant may also include a command for power control of the PUSCH. Additionally or alternatively, DCI may include power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH/EPDCCH may carry one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message may be transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
In LTE, the UL transmission scheduling command is transmitted from the eNB to the UE. There is a fixed delay between the time the scheduling command is transmitted and the time the UE transmits the UL signal specified in the standard. This delay is provisioned to allow the UE time to decode the PDCCH/EPDCCH and prepare the UL signal for transmission. For a FDD serving cell, this UL grant delay is 4 ms. For a TDD serving cell, this UL grant can be greater than 4 ms.
Carrier Aggregation
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One important requirement on LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of carrier aggregation.
FIG. 4 illustrates aggregated bandwidth by carrier aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. For example, a terminal may support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation in the ability to perform cross-carrier scheduling. This mechanism allows an EPDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the EPDCCH messages. For data transmissions on a given CC, a wireless device may expect to receive scheduling messages on the EPDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling. The mapping from EPDCCH to PDSCH is also configured semi-statically.
LTE Scheduling Methods
In LTE, the scheduling information of DL and UL transmission on the PCell is transmitted on the PCell using PDCCH or EPDCCH. This basic scheduling mechanism is referred to as the self-scheduling method in LTE. For a SCell two scheduling mechanisms are supported: SCell self-scheduling and SCell cross-carrier scheduling. According to SCell self-scheduling, the scheduling information of DL and UL transmission on the SCell is transmitted on the same SCell itself using PDCCH or EPDCCH. By contrast, according to SCell cross-carrier scheduling, the network can also configure a SCell via higher layer signaling. In this approach, the scheduling information of DL and UL transmission on a SCell is transmitted on a second cell using PDCCH or EPDCCH. The second cell may be the PCell or another SCell.
For LTE, the DL and UL scheduling approaches are configured together. Thus, the DL and UL transmissions of a cell are either both self-scheduling or both cross-carrier scheduling.
Wireless Local Area Network
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
When the range of several access points (APs) using the same frequency overlap, all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. FIG. 5 illustrates an example listen before talk (LBT) mechanism on a single unlicensed channel.
In the single-channel LBT case, after a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as Distributed Coordination function Inter-frame Spacing, or DCF Inter-frame Spacing, or DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it is required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
The Point Coordination Function Inter-frame Spacing, or PCF Inter-frame Spacing, or PIFS is used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it can be used by stations (STAs) operating under PCF, to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the PC shall sense the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the PC shall transmit a Beacon frame containing the CF Parameter Set element and a delivery traffic indication message element.
Load-Based Clear Channel Assessment
For a device not utilizing the Wi-Fi protocol, Europe Regulation EN 301.893, v. 1.7.1 provides the certain requirements and minimum behavior for the load-based clear channel assessment. FIG. 6 illustrates an example LBT mechanism in conformance with EN 301.893. The requirements and minimum behavior are as follows:                1. Before a transmission or a burst of transmissions on an operating channel, the equipment shall perform a CCA check by detecting the energy level of the operating channel. The equipment shall observe the operating channel(s) for the duration of the CCA observation time, which is set by the manufacturer and shall be not less than 20 μs. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in enumerated point #5 below. If the equipment finds the channel to be clear, it may send transmissions immediately (see point #3 below).        2. If during CCA check, the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that needs to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA (eCCA) is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer (see clause 5.3.1 q). The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit.         It should be noted that the equipment is allowed to continue Short Control Signaling Transmissions on this channel providing it complies with the requirements in clause 4.9.2.3.         For equipment having simultaneous transmissions on multiple (adjacent or non-adjacent) operating channels, the equipment is allowed to continue transmissions on other operating channels providing the CCA check did not detect any signals on those channels.        3. The total time that an equipment makes use of an operating channel is the maximum channel occupancy time which shall be less than (13/32)×q ms, with q as defined in point #2 above. After the maximum channel occupancy time, the device shall perform the extended CCA described in point #2 above.        4. Upon correct reception of a packet which was intended for the equipment, the equipment may skip CCA and immediately proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the maximum channel occupancy time as defined in point #3 above.         For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.        5. The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm e.i.r.p. transmitter the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz 4+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).Licensed-Assisted Access (LAA) to Unlicensed Spectrum Using LTE        
Up to now, the spectrum used by LTE has been dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited, and the allocated spectrum cannot meet the ever increasing demand for larger throughput from applications and/or services. Therefore, a new work item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
FIG. 7 illustrates licensed-assisted access (LAA) to unlicensed spectrum using LTE carrier aggregation. As depicted, a wireless device is connected to a primary cell (PCell) in the licensed band and one or more secondary cells (SCells) in the unlicensed band. Herein, a secondary cell in unlicensed spectrum may be referred to as a LAA secondary cell (LAA SCell). The LAA SCell may operate in downlink only mode or operate with both UL and DL traffic. Furthermore, certain embodiments may include LTE nodes operating in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi).
For LAA, the backoff counter does not have to be decremented when a slot is sensed to be idle during the ECCA procedure. Additionally, any subframe can be used for either DL or UL transmission.
To coexist fairly with the Wi-Fi system, transmission on the SCell must conform to LBT protocols in order to avoid collisions and causing interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration is specified by country and region-specific relations, e.g., 4 ms in Japan and 13 ms according to EN 301.893.
FIG. 8 illustrates LAA to the unlicensed spectrum with LBT and UL and DL transmissions within a transmission opportunity (TXOP). Specifically, in the example depicted, a 4 ms LAA TXOP after successful LBT consists of a DL transmission burst with two subframes followed by an UL transmission burst of two subframes. Thus, there is TXOP sharing between the downlink and the uplink. The UL burst may perform a single CCA, a short extended CCA, or no CCA before transmission.
UL Transmission in LAA
There may be two possible approaches to support UL transmission on an LAA SCell. In the first approach, the UE follows an LBT protocol to attempt channel access after receiving the UL transmission scheduling command. FIG. 9 illustrates UL LAA transmissions based on an UL LBT protocol. In the depicted example, the system has a 4 ms channel occupancy time. Thus, the LBT protocol is designed to allow 4 ms DL channel occupancy time and 4 ms UL channel occupancy time.
According to a second approach, the UE does not follow any LBT protocol to initiate channel access after receiving the DL transmission scheduling command. FIG. 10 illustrates UL LAA transmissions based on a reverse direction grant (RDG) protocol. In the depicted example, the system has an 8 ms channel occupancy time. Thus, the LBT protocol is designed to allow 8 ms total channel occupancy time between DL and UL transmissions. LBT and CCA are performed by the eNB before the start of DL transmissions.
Currently, there is no uplink control information (UCI) design for LTE operation on the unlicensed spectrum.